The invention relates to a roller vane pump used for operating an automatic transmission for motor vehicles and in particular for pumping automatic transmission fluid in a continuously variable transmission. The pump is provided with a pump housing, a rotor located in the pump housing and rotatable by means of a drive shaft, a cam ring located around said rotor and roller elements slideably accommodated with some tolerance in slots on the periphery of the rotor. On rotation of the rotor the roller elements interact in a sealing manner with the surface of the cam ring. The cam ring, the rotor, the roller elements and the pump housing define a number of pump chambers, which may arrive in communication with hydraulic channels in the pump housing for allowing flow of fluid to and from the pump chambers. Fluid is communicated between a hydraulic channel and a pump chamber either through one or more suction ports for allowing a predominantly axial flow of fluid to a pump chamber, or through one or more discharge ports for allowing a predominantly axial flow of fluid from a pump chamber.
Such a roller vane pump is known from the European patent 0.555.909 and is in particular adapted for pumping of large volumes of fluid particularly automatic transmission fluid, while maintaining a high pressure in a hydraulically controlled and operated continuously variable transmission for motor vehicles. In a continuously variable transmission of the belt-and-pulley type a large amount of fluid at a high pressure is needed to control the transmission ratio and the belt pinching force, even at a low engine speed. Since the pump is driven by a shaft drivingly connected to the engine shaft, the pump is designed to be able to provide a desired pump yield even at the lowest rotational speed of the engine.
When the pump is operated, the rotor rotates and a low pressure or suction pressure is effected in a pump chamber. Due to the suction pressure fluid is drawn from a hydraulic channel through a suction port or ports into a pump chamber. The flow of the fluid is dependent of said suction pressure and of the surface area of the suction port or ports. Inside a pump chamber, fluid is compressed and subsequently discharged through a discharge port to a hydraulic channel.
Although the known pump functions satisfactory per se, it possesses certain drawbacks. Both the amount of wear of pump parts and the level of noise generated by the pump are not optimal.
The aim of the invention is to optimise the known pump by reducing at least one of wear of pump parts and noise generated by the pump. This aim is, according to the insight underlying the present invention, achieved in providing for a modified rotor and/or a cam ring, the modification being such as to effect an increase of the suction pressure and/or a reduction of the pressure gradient. When a roller element, located in a slot on the periphery of the rotor, has just passed a discharge port, the fluid pressure in a pump chamber in front of that roller element has changed from a high discharge pressure to a much lower suction pressure. The difference between the two pressures is relatively large, as is the pressure gradient associated with said pressure change. Due to said pressure difference and since a roller element is fitted with some tolerance inside a slot, the roller element moves towards the front of the slot as seen in rotational direction of the rotor, where it collides with the rotor generating noise and resulting in wear of the element and of the rotor. Furthermore, inside the known pump the suction pressure becomes low enough for cavitation to occur even at generally occurring pump parameters. Cavitation amounts both to wear of pump parts and to noise generated by the pump, as is commonly known. A pump according to the invention has an improved functionality, since its functional life is prolonged and less noise is generated by the pump during operation. A rotor and a cam ring according to the invention can be adopted both simultaneously and alternatively.